The limited processing capacity of the human visual system is clear whenever we look at a crowded or cluttered scene. We have all experienced the difficulty in finding the face of a friend in a crowd. Despite our familiarity with the face we are looking for, such a search is almost always a slow and serial one; we cannot process all the faces at once. In the proposed set of experiments we plan to investigate the neural underpinnings of this limited processing capacity. Why is it that we can process only a limited number of items at a time? The biased competition theory of selective attention posits one explanation for this limitation. Both single-cell and fMRI studies have shown that when multiple nearby stimuli appear simultaneously in the visual field, they are not processed independently, but interact in a mutually suppressive way. Moreover, this competition has been shown to occur most strongly at the level of the receptive field (RF). Processing capacity limitations, by this theory then, are a consequence of competition among stimuli for neural representation at a sensory level. However, much of the behavioral literature on processing capacity limitations, have considered processing limitations to be a consequence of a limited attentional capacity; that is, a limit in our ability to direct attention towards multiple items, a function generally associated with the frontoparietal cortex. The objective of the proposed experiments is to use fMRI to investigate whether neural competition in the visual cortex can explain the limited processing capacity of the visual system or whether limited processing capacity is better reflected in the activity of the frontoparietal cortex and our ability to direct attention to multiple locations. The experiments proposed here should further our understanding of the interaction of visual processes and attention, as well as provide a deeper understanding of the neural basis of our limited processing capacity. Understanding the neural mechanism that constrain the number of visual items we can process effectively at any one time may have important implications in treating a variety of behavioral and brain disorders that result in diminished attentional capacity, including attention deficit disorder, Alzheimer's disease, and unilateral neglect syndrome. [unreadable] [unreadable] [unreadable]